The present invention relates to an antenna used for portable devices, such as cell phones.
In recent years, portable devices such as cell phones have been widely diffused. These devices are now designed in more and more compact size. In association with this trend, antennas used for such devices are also designed in compact size, so that the antennas can be retracted in the case of the device.
A xcex/4 monopole antenna is commonly used with portable devices. The antenna of portable devices is extended from the case to secure a high gain when it is being used. The antenna is usually retracted in the case when the device is not in use so that it does not bother the user. Even when it is not in use and the antenna is held in the case, the user must be always ready to respond to calls. In this respect, the antenna must have such a gain that an ID can be transmitted and a call signal can be received.
FIG. 1 shows cross-sectional views of a conventional type mobile device provided with a xcex/4 monopole antenna. FIG. 1(a) shows the condition when the antenna is extended, and FIG. 1(b) shows the condition when the antenna is retracted in the case of the device.
In these figures, reference numeral 1 represents a case of a portable device; 2 a xcex/4 antenna element; 3 a knob made of conductive or insulating material; 4 an RF power feed connection; 5 an insulating tube; and 6 a radio frequency (RF) power source or a tuner.
The antenna of the portable device is switched over to connect to the RF power source or the tuner. For simplicity, the invention is not also described below, for all embodiments, with reference to a connection to a tuner.
When the portable device is in operation, an RF power source 6 is always connected to the RF power feed connection 4. The antenna element 2 is designed in such manner that it can be extended from or retracted into the case 1 of the portable device while it is kept in contact with the RF power feed connection 4, and it is constantly connected to the RF power source 6 via the RF power feed connection 4.
When the portable device is used, the antenna element 2 is extended from the insulating tube 5 in the case, as shown in FIG. 1(a). When the device is not in use, the antenna element 2 is retracted and accommodated in the insulating tube 5 in the case, as shown in FIG. 1(b). In both the extended condition shown in FIG. 1(a) and the retracted condition shown in FIG. 1(b), the antenna element 2 works as a xcex/4 monopole antenna.
The conventional type xcex/4 monopole antenna, as described above, works as a xcex/4 monopole antenna regardless of whether it is in the extended condition or in the retracted condition. As a result, it does not always have a sufficiently high gain, which is an important performance characteristic for mobile devices.